Excel 2007

When I start to enter a function into a worksheet cell, there is a dropdown box
that appears with a list of all the possible functions starting with that
letter (letters).

The same holds true if the function is a UDF that I have written in that
particular workbook.

It also occurs if the function is part of the morefunc.xll add-in.

However, it does not occur if the function is a UDF coming from an add-in -- at
least if the add-in is an .xla file.

Any solutions?
--ron